These medications may not necessarily poisonous, but people prolong their illnesses by choosing these medicines in place of scientifically-supported medicines.
That depends entirely on what they are buying and what they are using it for. Again, the consumer should ultimately be doing their own research about anything they buy over the counter, "natural" or not, instead of trying to solicit medical advice from low-level customer service employees.
They should, but they won't, for various reasons.
For example, many people think that anecdotal evidence is a good basis for making decisions about their healthcare. This not only applies to naturopathy, chiropractic etc. but also the proliferation of unqualified diet gurus.
They have also swallowed the 'if it's natural then it's safe' message, where the implication are that mainstream pharmaceuticals are all harmful, and that natural remedies are not useless.